The present invention is related to controlling the linear movement of a web or tape between spools or reels and particularly to controlling the transport of ribbon past a print head in connection with impact printing of symbols on a record medium.
This application is related to my copending commonly assigned U.S. patent application Ser. No. 399,129 filed July 16, 1982 and Ser. No. 399,216 filed July 19, 1982 and Ser. No. 399,130 filed July 16, 1982 and Ser. No. 504,959 filed June 16, 1983. The disclosure of these related applications is hereby incorporated by reference.
A common form of linear speed control for a moving web employs a constant speed capstan pinch roller drive, as used, for example, in tape or ribbon transports. In such transports where constant tape velocity is required, a special drive is required for the takeup reel and the payout reel as well as for the pinch roller drive in order to attain reasonable precision for the surface velocity of the tape. Added complications arise when the web has to be driven bidirectionally at high speed. Heretofore the use of a pinch roller has provided certain disadvantages, as for example, problems in maintaining the proper tension in the ribbon during movement, and the problem of proper tracking particularly where the ribbon is of fairly broad width.
Accordingly, one object of this invention is to provide an improved control for providing constant surface velocity of a moving web or tape.
A further object of this invention is to provide a tape drive requiring a lesser number of mechanical components and providing improved linear velocity control.
A further object of this invention is to provide a linear velocity control for use with an inked ribbon having a large width.
Briefly in accordance with one embodiment of the invention, step motors and a digital controller are employed to eliminate the capstan drive. By making use of the feedback pulses emitted from the payout step motor as it is pulled during tape movement, a closed loop digital system is provided to regulate the tape speed with sufficient accuracy for many applications as for example in high speed impact printing. Speed regulation is obtained by processing the feedback pulses to provide drive step pulses for a takeup step motor whose step rate is a function of the feedback pulse rate. The digital control uses a function table which is contained in a read only memory. This read only memory is addressed by the number of feedback pulses emitted during a sample number of takeup step pulses. The sample number and function table entries are determined by the accuracy and resolution requirements. While the control logic for this system might be implemented with common logic elements, that is gates, counters, etc., in one embodiment to be disclosed herein a microprocessor is provided.